1. Field of the Invention
The present invention relates to a battery module system, a charging type vacuum cleaner using the battery module system and a method of charging the battery module.
2. Description of the Related Art
Secondary batteries having high energy densities are developed and utilized as power sources for miniature information devices such as cellular telephones and portable personal computers (PCs). These secondary batteries are used in such a manner that the number of secondary batteries (unit cell) constituting a battery module and the way of combining series-connection and parallel-connection are varied according to voltage and current required for each device. Because the power source voltage of the above miniature portable devices is about several volts to 10 volts, a secondary battery is singly used or about 2 to 3 series of batteries are mostly used even if a battery module is used in which plural secondary batteries are connected in series. However, in recent years, the applications of secondary batteries are not confined to information devices but have been rapidly spread towards high-power and high-voltage fields such as domestic electric appliances, power tools, electric mopeds and hybrid cars. Along with this, the number of series in a battery module is increased and it is no rare case that 10 or more secondary batteries are connected in series upon use.
A problem arising when batteries are connected in series is variations between unit cells. The variation involves those observed from various viewpoints such as variations in capacity, impedance and SOC (state of charging). There are variations in voltage in a full charge state as a problem leading, particularly, to disorders. When batteries different in capacity are connected in series or in the condition that the SOCs (state of charging) of them are deviated from each other, unit cells having a voltage higher than the average and unit cells having a voltage lower than the average arise in a full-charged condition of the battery module, the unit cell having high voltage is put into a an overcharged state and is resultantly deteriorated greatly. If such charging is repeated, the unit cell that is largely deteriorated by the overcharging is reduced in capacity and is further overcharged, so that the progress of the deterioration of the unit cell is accelerated. As a result, this gives rise to the problem that the cycle life of the battery module is shortened more significantly than the unit cell.
To deal such a problem, a method is usually adopted in which charging called electrical reconditioning is carried out to eliminate variations in voltage in a charged state in a battery module of a nickel metal-hydride battery. The nickel metal-hydride battery has the performance that if it is intended to further charge a nickel metal-hydride battery in a state close to a full charge state, a charging reaction of the electrode material and a decomposition/regeneration reaction of the water in the electrolytic solution compete with each other, so that the charge reaction does not proceed. Therefore, if overcharging is carried out in such a proper charging condition as to prevent deterioration in the performance of the battery, the charging voltages of the batteries connected in series can be uniformed by utilizing an electrochemical current bypass function of the battery. There are many known examples of such an electrical reconditioning method, for example, JP-A 2001-314046 (KOKAI).
Generally, the coulomb efficiency of charging and discharging is almost 100% in secondary batteries and capacitors using a nonaqueous electrolyte and therefore, unlike the nickel metal-hydride battery, the current bypass function by the battery cannot be expected. To deal with such a case, a method is proposed in which an equalizing circuit in which each cell is bypassed is disposed as an external circuit to bypass charging current for unit cells charged to voltages exceeding a predetermined value, thereby suppressing variations in charge voltage. For example, JP-A 2002-238179 (KOKAI) discloses such technologies that in a battery module provided with plural unit cells connected in series, a Zener diode is connected to each unit cell in parallel to bypass charging current for unit cells charged to voltages exceeding the Zener voltage.
However, even if such a method is adopted, it is difficult to efficiently eliminate variations in the charged voltage of a unit cell because of the following problems. First, in the case of intending to attain the necessary function by a single element such as a Zener diode, the charge voltage of the battery is governed by variations in Zener voltage. It is difficult to suppress variations in Zener voltage like the case of producing a battery reduced in variation. The rise of Zener current when the charge voltage reaches the Zener voltage is by no means steep and therefore, bypass current starts flowing at a voltage lower than the required charge voltage. It is thus difficult to apply this method to a secondary battery for which the voltage must be controlled on the order of several tens of millivolts. To mention other problems, when the capacity of a battery module is large or the charge current is large in the case of, for example, carrying out rapid charging, power consumption in a Zener diode is increased, which makes it difficult to actually practice this method due to problems such as heat generation. In order to evade problems such as those mentioned above, the bypass circuit is constituted not by a single element such as a Zener diode but by a bypass circuit which comprises standard voltage and feedback control and to which a large-current switching element is applied, which makes it possible to put this method into practice on principle. However, actually, if the number of series of cells in the battery module is large, the circuit is complicated and also, the size and cost are made to be too large for a battery module, which makes it difficult to realize this circuit.
When the above bypass control circuit is made collectively into a compact form using ICs, this circuit may be applied to even a battery module increased in the number of series. On the other hand, such a bypass circuit does not allow large bypass currents to flow, and therefore only very slow equalizing control is permitted. When, particularly, unit cells having the large voltage variation rate with the capacity in the vicinity of full charge voltage are used, variations in voltage is rapidly increased with the progress of charging, which excessively increase the bypass current required to expect that the bypass will restrict the variation in voltage. It is therefore very difficult to restrict variations in the charge voltage of unit cells only by a bypass circuit.